Genomic analyses of the Linum distyly supergene reveal convergent evolution at the molecular level.

Supergenes govern multi-trait-balanced polymorphisms in a wide range of systems; however, our understanding of their origins and evolution remains incomplete. The reciprocal placement of stigmas and anthers in pin and thrum floral morphs of distylous species constitutes an iconic example of a balanced polymorphism governed by a supergene, the distyly S-locus. Recent studies have shown that the Primula and Turnera distyly supergenes are both hemizygous in thrums, but it remains unknown whether hemizygosity is pervasive among distyly S-loci. As hemizygosity has major consequences for supergene evolution and loss, clarifying whether this genetic architecture is shared among distylous species is critical. Here, we have characterized the genetic architecture and evolution of the distyly supergene in Linum by generating a chromosome-level genome assembly of Linum tenue, followed by the identification of the S-locus using population genomic data. We show that hemizygosity and thrum-specific expression of S-linked genes, including a pistil-expressed candidate gene for style length, are major features of the Linum S-locus. Structural variation is likely instrumental for recombination suppression, and although the non-recombining dominant haplotype has accumulated transposable elements, S-linked genes are not under relaxed purifying selection. Our findings reveal remarkable convergence in the genetic architecture and evolution of independently derived distyly supergenes, provide a counterexample to classic inversion-based supergenes, and shed new light on the origin and maintenance of an iconic floral polymorphism.

MicroRNA annotation of plant genomes - Do it right or not at all.

MicroRNAs are non-coding regulators of gene expression and key factors in development, disease, and targets for bioengineering. Consequently, microRNAs have become essential elements of already burgeoning draft plant genome descriptions where their annotation is often particularly poor, contributing unduly to the corruption of public databases. Using the Citrus sinensis as an example, we highlight and review common failings of miRNAome annotations. Understanding and exploiting the role of miRNAs in plant biology will be stymied unless the research community acts decisively to improve the accuracy of miRNAome annotations. We encourage genome annotation teams to do it right or not at all.